The present invention relates generally to an ultra-sonic distance sensor for measuring the distance to an object capable of reflecting ultra-sonic waves by measuring the elapsed time between transmission of the ultra-sonic wave and receipt of reflected ultra-sonic waves. More specifically, the invention relates to an ultra-sonic distance sensor system which can correct the sensor value with a correction value derived on the basis of reflected signal conditions.
Published Japanese Patent Application (Tokkai) Showa 56-153268, published on Nov. 27, 1981 discloses an ultra-sonic distance sensor which measures distance by means of ultra-sonic waves. The system disclosed allows the sensor to range objects over a much wider angular area. A similar system has been disclosed in Published Japanese Patent Application (Tokkai) Showa 56-153267, published on Nov. 27, 1981, in which an estimated maximum distance can be preset to determine discharge or transmission timing of the ultra-sonic wavepulses or packets.
Published Japanese Patent Application (Tokkai) Showa 57-182544 and Published Japanese Utility Model Application (Jikkai) Showa 55-141085 disclose an ultra-sonic distance sensor applied for detecting road clearance in front of a vehicle. Application of an ultra-sonic distance sensor to a height-control system of an automotive vehicle has also been disclosed in the European Patent First Publication No. 0 091 017, published on Oct. 12, 1983, which corresponds to the co-pending U.S. patent application Ser. No. 476,519 filed Mar. 18, 1983 now U.S. Pat. No. 4,518,169.
Generally, an ultra-sonic distance sensor measures the elapsed time between transmission of an ultra-sonic wave packet and receipt of ultra-sonic waves reflected by the object to be ranged. Receipt of the reflected ultra-sonic waves is recognized when the signal level of the reflected ultra-sonic wave receiver signal rises across a given threshold. Assuming the reflected ultra-sonic wave receiver signal is in the form of a rectangular wave, the higher signal level of which corresponds to the maximum amplitude of the reflected ultra-sonic waves, no deviation between the measured distance and an actual distance would be included in the resultant value. However, in practice, the ultra-sonic wave receiver signal is generally in the form of a sine wave in which the signal level increases and decreases gradually. The rate of change of the signal level depends on the amplitude of the reflected ultra-sonic waves. For instance, when the amplitude of the reflected ultra-sonic waves remains at approximately the same level as that of the transmitted ultra-sonic waves the rising edge of the receiver signal is steep enough to prevent any deviation between the measured distance and the actual distance. On the other hand, when the ultra-sonic wave is partially diffused or absorbed by the surface of the object and thus the amplitude of the reflected ultra-sonic wave is correspondingly reduced, the rising or leading edge of the receiver signal will be rather shallow, so that the deviation between the measured distance and the actual distance may become significant and cannot be disregarded when precise measurement of the distance is necessary.